I. Field of the Invention
The present invention relates to transmissions. More particularly, the present invention relates to multi-speed rear wheel drive transmissions.
II. Description of the Prior Art
Typically, most rear wheel drive transmissions are constructed with a short input shaft which transmits driving torque from a source such as an engine through a pair of gears known as a headset, to a countershaft. The countershaft is located parallel to the input shaft and is positioned amid a plurality of driving gears. An output shaft, having a plurality of driven output gears surrounding the shaft, is located parallel to the countershaft. Each of the driven output gears surrounding the output shaft is in mesh with a corresponding driving gear from the plurality of gears on the countershaft. Usually the output shaft is coaxial with the input shaft. A number of axially reciprocating synchronizers are coupled to the output shaft or countershaft to engage one of the speed gears on one side and another of the speed gears on its other side. One of the speed gears on the output shaft is a reverse gear which is in mesh with the driving gear on the countershaft through an idler gear. Most often, the output shaft is coaxial with the input shaft with one of the synchronizers arranged to engage, in one position, the input shaft directly to the output shaft to effect one of the speed changes.
With the typical headset multi-speed transmission, all of the driving and driven speed gears are in continuous motion when the vehicle is stationary, the transmission is in neutral, the driving source, or engine, is running and the clutch is engaged. Invariably, the driving source, or engine, generates angular accelerations in the power output characteristics that induce rotational harmonics of the drive train. The rotational harmonics of the rotating gears of the typical headset multi-speed transmission cause a considerable noise problem commonly referred to as "neutral roll-over noise".
With the typical headset transmission design, the torque of the driving source or engine is multiplied by the headset gear ratio. Hence, all of the speed gears that transmit torque in the power flow sequence after the headset must have an adequate face width to transmit the multiplied torque. Since the torque multiplication is transmitted through a single driving gear on the countershaft to an engaged driven gear on the output shaft, the countershaft and output shaft have to be sized and supported to withstand considerable deflection forces.
When shifting gears in a conventional headset transmission, the activated synchronizer must speed up or slow down the revolutions (rpm) of all of the speed gears, countershaft, input shaft and clutch disc. The shift sequence, first to second, third to fourth, third to second, etc. determines the magnitude of the rpm changes affected at the speed gears, countershaft, input shaft and clutch disc. The rpm changes of these rotating masses create a different amount of reflected inertia at the activated synchronizer, which results in the need of different synchronizer sizes to produce an acceptably low shift effort. Recently, double and triple cone synchronizers have been used to reduce shift effort.
Vehicles equipped with a typical headset multi-speed transmission cannot be towed without restrictions and/or the risk of serious damage to the transmission when the rear wheels of the vehicle are in contact with the pavement, the transmission is in neutral and the drive train clutch is engaged to the stationary driving source or engine. Under these conditions, the rotation of the output shaft may cause serious damage to bearings, journals, or thrust surfaces, since the rest of the transmission components are in a stationary state and adequate lubrication of the bearings, journals, gear meshes, or thrust surfaces does not occur. To avoid such damage, towing under these conditions is typically restricted to speeds of no greater than 30 miles per hour and for distances of 50 miles or less.